The Heating Components of a Furnace

Furnaces work in a cycle, warming cold air before distributing it throughout the house.

Furnaces heat cooler air from inside the home and send the warmed air back into living spaces. A furnace ignites a fuel source -- such as natural gas, propane or oil -- to produce the energy required to warm the air. Most systems in the United States use either natural gas or propane. Homes in colder climates, such as those found in the Northeast, often rely on oil-burning furnaces. Blowers funnel the warm air through ducts that travel to each room in the house.

Igniter

Older gas systems used a pilot light to ignite natural gas or propane. The pilot light burned constantly and was at the ready when the thermostat called for heat. Modern furnaces have replaced the pilot light with a glow stick made of silicone nitride. The glow stick only comes on when the thermostat calls for heat. Oil furnaces use an igniter similar to the glow sticks used in gas furnaces.

Burner

The burner is where fuel mixes with air to create combustion. Small quantities of gas and air enter the chamber as the thermostat calls for heat. The igniter or pilot light creates an ignition source that burns the gas. In oil burners, the oil is sprayed into the burner as a fine mist, which blends with air and ignites. The burner is also known as a combustion chamber.

Heat Exchanger

The heat exchanger is located just above the burner. The heat generated from burning fuel warms the colder air inside the heat exchanger. When the temperature reaches a preset level, a blower kicks on and distributes warm air through the duct system. The blower continues to run even after the combustion sequence ends so it can distribute all the warm air left inside the heat exchanger.

Exhaust Gases

A flue vents exhaust gases from the combustion process to the outside. Some newer, high-efficiency furnaces have a second combustion chamber. This unit captures some of the gases from the flue, compresses them and burns them a second time to squeeze out every bit of energy they hold.

Plenum

A plenum collects cooler air from the home as the furnace cycles. The plenum is situated close to the heat exchanger. Pressure differences inside the furnace system force the air in the plenum to become heated before it passes through the duct system.

About the Author

Robert Korpella has been writing professionally since 2000. He is a certified Master Naturalist, regularly monitors stream water quality and is the editor of freshare.net, a site exploring the Ozarks outdoors. Korpella's work has appeared in a variety of publications. He holds a bachelor's degree from the University of Arkansas.